This invention relates to injection molding and more particularly to an improved electrically heated sprue bushing and method of making the same.
The sprue bushing of the present invention represents an improvement over the sprue bushing disclosed in the applicant's previous U.S. patent application Ser. No. 36,880 filed May 8, 1979. It relates to the same type of structure with a helical heating coil embedded in a highly heat conductive material cast over an inner core portion formed of a corrosive resistant material, but it also includes a finished outer sleeve portion. The previous sprue bushing has the disadvantage that its outer surface is formed by the cast material. This requires that the outer surface of each sprue bushing be machined to provide the necessary smooth finish. Furthermore, the highly conductive material which is usually a beryllium copper alloy does not have sufficient corrosion resistance to provide a durable outer casing of a sprue bushing. This has resulted in it requiring a costly nickel plating process to protect it from corrosive gases escaping from the gate area.
The process of making the applicant's previous sprue bushing therefore has the serious disadvantages that costly machining and plating steps are required after casting. Furthermore, the even application of heat along the runner passage is critical and therefore it is important that there be optimum heat transfer between the heating element and the surrounding conductive material and between the conductive material and the inner core portion through which the runner passage extends. Resistance to heat transfer at the interfaces between the different materials results in uneven temperature distribution which may shorten the life of the heating element and may cause the melt to deteriorate.